The present invention relates to signal measurement systems and more particularly to phase difference measurement systems using reference and variable phase signals.
In the prior art, there are a variety of systems which employ phase difference measurement techniques to determine certain characteristics of the system. In such systems, a variable input signal is generally provided which reflects changing system characteristics by producing a change in the phase of that input signal. A reference signal, which maintains a constant phase, is then compared with the variable phase signal in a comparator circuit to produce a phase difference which is directly proportional to the changing characteristic of the system. By measuring the phase difference of the variable input signal with respect to the reference signal, the magnitude, rate of change or other calculation may be determined with respect to the selected characteristic.
By way of example, in one type of aviation navigation system known as the very high frequency omnidirectional range (VOR) technique, a phase difference measurement is made to provide an indication of aircraft bearing with respect to ground positions for enabling aircraft navigation. This particular system is conventional and is currently utilized for aircraft navigation throughout the world. As is known, the system includes a plurality of ground stations positioned at various points throughout each country with each of the stations precisely located on aircraft navigation maps. By determining the location of an aircraft with respect to any one or more of the ground stations, the exact location and course of that aircraft can be accurately fixed.
In one type of VOR system, the VOR ground station includes an omnidirectional transmitter which operates in the carrier frequency range of 108 to 118 MHz and transmits two 30 Hz modulating signals. One of the 30 Hz signals is an FM omnidirectional reference signal which is constant regardless of the radial position of an aircraft with respect to the VOR ground station. The other 30 Hz signal is a variable AM signal which has its phase shifted with respect to the reference signal by an amount dependent upon the angular location of an aircraft with respect to the VOR ground station. The system is designed to allow a measurement of the radial position of an aircraft from the ground station with respect to magnetic north by determining the phase difference between the variable 30 Hz AM signal and the 30 Hz FM reference signal. This phase difference can be converted directly into a bearing indication or used to control a VOR indicator or other display device capable of providing indications of aircraft bearing.
Originally, phase differences in any phase difference measurement system were detected using analog devices and displayed by mechanical or electromechanical instruments. With the advent of digital circuits and computer technology, however, many of the old mechanical and electromechanical systems are being replaced with solid state circuitry. While the advances in technology have provided improved accuracy, reduced weight, lower power consumption, and a reduction in the physical size of the system, other difficulties in converting to digital formats have been encountered that were not observed in prior analog systems. In addition, as many aircraft systems are converted to digital implementations, there is a trend to employ microprocessor-based instrumentation which provides highly efficient and accurate digital calculations but which is limited in size and speed over larger systems. There is therefore a continuing need to improve the operating speed and accuracy of such microprocessor-based instrumentation systems without requiring significant increases in the digital circuitry.
In one phase difference measurement system disclosed in U.S. patent application Ser. No. 435,517 entitled "A Frequency, Amplitude Independent Phase Difference Measurement Technique", filed Oct. 20, 1982 by Thomas E. Gehrke, and assigned to the same assignee as the present invention, an improved technique is disclosed which produces in-phase and quadrature phase signals useful for providing phase difference measurements. In the referenced system, one of the input signals is provided to a phase-locked loop which locks onto the input signal and uses the output of a voltage controlled oscillator to derive the sine and cosine of the input signal. The sine and cosine are then 90.degree. out of phase and form in-phase and quadrature phase signals with equal amplitudes which are used to derive the sine and cosine of the phase shift between the variable and reference signal. Although this technique may be successful in producing accurate determinations of phase difference in the phase difference measurement system, there is still a need for alternative systems for use in different applications.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide an improved digital system for providing phase difference calculations, particularly in connection with VOR bearing measurements.